Effect of Welding Current on Weld Formation, Microstructure, and Mechanical Properties in Resistance Spot Welding of CR590T/340Y Galvanized Dual Phase Steel

materials

Article Effect of Welding Current on Weld Formation, Microstructure, and Mechanical Properties in Resistance Spot Welding of CR590T/340Y Galvanized Dual Phase Steel

Xinge Zhang 1,2, Fubin Yao 3, Zhenan Ren 2,* and Haiyan Yu 4

1 School of Mechanical and Aerospace Engineering, Jilin University, Changchun 130025, China; [email protected] 2 College of Materials Science and Engineering, Jilin University, Changchun 130025, China 3 Doosan Infracore China Co., Ltd., Yantai 264006, China; [email protected] 4 Sanyou Automobile Parts Manufacturing Co., Ltd., Changchun 130022, China; [email protected] * Correspondence: [email protected]; Tel.: +86-431-8509-4376

Received: 20 September 2018; Accepted: 13 November 2018; Published: 17 November 2018

Abstract: During resistance spot welding, the welding current is the most important process parameter, which determines the welding heat input and then has a great inﬂuence on the welding quality. In present study, the CR590T/340YDP galvanized dual phase steel widely used as automobile material was carried out using resistance spot welding. The effect of welding current on the weld formation, microstructure, and mechanical properties was studied in detail. It was found that the quality of weld appearance decreased with the increase of welding current, and there was a Zn island on the weld surface. The microstructure of the whole resistance spot welded joint was inhomogeneity. The nugget zone consisted of coarse lath martensite and a little of ferrite with the columnar crystal morphology, and the microstructure of weld nugget became coarser when the welding current was higher. There was an optimum welding current value and the tensile strength reached the maximum. This investigation will provide the process guidance for automobile body production.

Keywords: Resistance spot welding; galvanized dual phase steel; microstructure; mechanical properties; weld formation

1. Introduction In the automotive field, reducing the body weight is one of the most fundamental ways to save energy and reduce pollution. Therefore, more and more lightweight and high-strength materials have been developed for body manufacturing [1–3]. Galvanized dual phase steel has the advantages of low yield ratio, good formability, high tensile strength, good matching of strength and plasticity, corrosion resistance, and so on, which has been used in automobile body manufacturing [4–6]. The wide application potential of galvanized dual phase steel in the automotive field mainly depends on the welding methods and its welding quality. In previous literature, several welding methods, such as laser welding [7–10], gas metal arc welding (GMAW) [11–13], and friction stir welding (FSW) [14–16], have been employed to weld the galvanized dual phase steel, and these welding methods have their own advantages and disadvantages, as well as being applicable to different base metal materials and joint shapes. Resistance spot welding has the advantages of high efficiency and low cost, which is an important welding method for manufacturing the automobile sheet structures [17,18]. Many scholars have studied the microstructure, softening zone characteristics, mechanical behavior, and welding spatter defect of resistance spot

Materials 2018, 11, 2310; doi:10.3390/ma11112310 www.mdpi.com/journal/materials Materials 2018, 11, x FOR PEER REVIEW 2 of 13 Materials 2018, 11, 2310 2 of 13 studied the microstructure, softening zone characteristics, mechanical behavior, and welding spatter defect of resistance spot welding of dual phase steels [19–23]. The CR590T/340YDP galvanized dual weldingphase steel of dual sheet phase used steels as an [19 automobile–23]. The CR590T/340YDP manufacturing material galvanized not dual only phase has steelgood sheet corrosion used asresistance, an automobile but also manufacturing high strength, which material can not effect onlyively has reduce good the corrosion weight of resistance, the car body, but alsoand some high strength,research on which resistance can effectively spot welding reduce have the been weight carried of the out car [24–26]. body, andWang some et al. research [27] investigated on resistance the spoteffect welding of base material have been chemical carried compositions out [24–26]. Wang on the et properties al. [27] investigated of the resistance the effect spot ofwelding base material joint of chemicalDP590 steel, compositions and the results on the indicated properties the of thetensile resistance strength spot and welding toughness joint ofof DP590welded steel, joints and were the resultsaffected indicated by the chemical the tensile compositions strength and of toughnessthe base material, of welded especially joints were the carbon affected content. by the chemicalNamely, compositionsfor the same grade of the DP590 base material, steel, the especially weld formation, the carbon microstructure, content. Namely, and for mechanical the same gradeproperties DP590 of steel,resistance the weld spot formation,welding will microstructure, be different if andthe chemical mechanical compositions properties ofof resistancethe base material spot welding are not will the besame. different During if thethe chemicalresistance compositions spot welding of process, the base it materialis well known are not that the the same. welding During heat the generation resistance 2 spotcan be welding expressed process, as Q it= isI2Rt well (I knownis the welding that the current; welding R heat is the generation resistance; can and be t expressed is the welding as Q =time).I Rt (InI is general, the welding the welding current; timeR andis the welding resistance; current and aret 10is− the1 s level welding and kA time). level, In respectively. general, the Therefore, welding −1 timethe welding and welding current current is considered are 10 ass thelevel key and factor kA level,to determine respectively. the welding Therefore, heat the input welding and influence current isof consideredthe welding as quality the key [28]. factor Wang to determineet al. [29] theestablished welding a heatfinite input element and model influence for ofresistance the welding spot qualitywelding [28 of]. DP590Wang steel, et al. and [29] theestablished nugget formation a finite element process model was investigated. for resistance The spot simulative welding ofresult DP590 for steel,nugget and size the was nugget obviously formation bigger process than that was of investigated.the experiment The under simulative a large resultwelding for current, nugget although size was obviouslythey were biggerwell fitted than under that of a thesuitable experiment welding under curren a larget. Therefore, welding the current, experimental although investigation they were well on fittedthe effect under ofa the suitable welding welding current current. on properties Therefore, in the resistance experimental spot investigationwelding of DP590 on the steel effect has of thean weldingimportant current significance. on properties in resistance spot welding of DP590 steel has an important significance. ToTo exploreexplore the the weldability, weldability, and provideand provide the process the process with guidance with guidance for automobile for automobile body production, body theproduction, CR590/340Y the CR590/340Y galvanized dualgalvanized phase steeldual sheetsphase employedsteel sheets for employed the automobile for the front automobile longitudinal front beamlongitudinal part were beam carried part were out by carried resistance out by spot resistance welding, spot and welding, the influence and the of influence the welding of the current welding on weldcurrent formation, on weld formation, microstructure, microstructure, microhardness, microhardness, and tensile and strength tensile was strength studied was in detail.studied in detail.

2. Materials and Methods 2. Materials and Methods In this study, the base metal is galvanized dual phase steel (Trademark: CR590T/340Y) sheet, In this study, the base metal is galvanized dual phase steel (Trademark: CR590T/340Y) sheet, whichwhich isis alwaysalways usedused toto manufacturemanufacture thethe automobile’sautomobile's frontfront longitudinallongitudinal beambeam part.part. The thickness of thethe basebase metalmetal isis 22 mm,mm, andand itsits .hemical.hemical compositionscompositions areare listedlisted inin TableTable1 1.. The The XRD XRD pattern pattern result result indicatesindicates thatthat thethe basebase metalmetal consistedconsisted ofof ferriteferrite (86.2(86.2 vol.%) and martensite (13.8(13.8 vol.%), as shown in FigureFigure1 1.. TheThe galvanizedgalvanized dualdual phasephase steelsteel sheetssheets were were conducted conducted with with double-side double-side hot hot galvanizing galvanizing and the weight of the Zn layer is 80 g/m3. The yield strength and tensile strength of the base metal are and the weight of the Zn layer is 80 g/m3. The yield strength and tensile strength of the base metal 356are 356 MPa MPa and and 605 MPa,605 MPa, respectively. respectively.

Table 1. Chemical compositions of base metal (wt.%). Table 1. Chemical compositions of base metal (wt.%). Component C Si Mn S P Al Cr Nb Mo Fe Component C Si Mn S P Al Cr Nb Mo Fe wt% 0.087 0.087 0.0080.008 1.6941.694 0.0070.007 0.0010.001 0.0300.030 0.1600.160 0.0120.012 0.0110.011 BalanceBalance

FigureFigure 1.1. XRD pattern of base metal.

TheThe resistanceresistance spotspot weldingwelding robotrobot (Shougang(Shougang MOTOMANMOTOMAN RobotRobot Co.,Co., Ltd.,Ltd., Beijing,Beijing, China)China) waswas employedemployed toto weldweld thethe overlap joints, and the workingworking face diameter of Cu-Cr-Zr alloy electrodes was

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66 mm. mm. The The dimensions dimensions and and configuration conﬁguration of of the the joint joint are are indicated indicated in in Figure Figure 22.. The welding time and electrodeelectrode force force were were set set at at 20 cycles and 4.0 kN by preliminary process experiments, and and the the main main experimentalexperimental parameters parameters are are given in Table 2 2..

FigureFigure 2. 2. SchematicSchematic diagram diagram of of dimensions dimensions and and config conﬁgurationuration of of resistance resistance spot spot welding welding joint.

Table 2. Experimental parameters. Table 2. Experimental parameters.

ParameterParameter Welding Current (kA) Welding Welding Time Time (C (Cycle,ycle, 1 1 Cycle Cycle = = 0.02 0.02 s) s) Electrode Electrode Force Force (kN) (kN) ValueValue 8.5~12.0 20 20 4.0 4.0

After welding, the appearance of the resistance spot weld was observed by digital camera (Canon Ixus1000, Cannon,After Tokyo, welding, Japan). theThe appearanceresistance spot of weld the was resistance cut from spotweld weldnugget was center, observed and then bymounted, digital polished, camera and(Canon etched Ixus1000, for the microstructure Cannon, Tokyo, observed Japan). and The analysis. resistance The spot etching weld solu wastion cut was from 4 vol.% weld nitric nugget acid. center, The microstructureand then mounted, of base polished,metal, heat and affected etched zone, for and the weld microstructure nugget was examined observed by and Evol-18 analysis. scanning The electron etching microscopysolution was (SEM) 4 vol.% (Carl nitric Zeiss, acid. Jena, TheGermany) microstructure with energy of dispersive base metal, spectr heatoscope affected (EDS). zone, The and phase weld structures nugget werewas examinedidentified using by Evol-18 the X-ray scanning diffractometer electron microscopy(XRD) (D/MAX-2500PC, (SEM) (Carl Rigaku, Zeiss, Jena,Tokyo, Germany) Japan) machine with energy with 50kVdispersive voltage, spectroscope 300 mA current, (EDS). Cu K Theα radiation, phase structuresand 4 °/minwere scanning identiﬁed rate. The using Vickers the hardness X-ray diffractometer was measured on(XRD) the cross (D/MAX-2500PC, section of the welded Rigaku, joint Tokyo,using an Japan) MH-3 microhardness machine with test 50kV machine voltage, (Shanghai 300 mA Tuming, current, Shanghai, Cu Kα ◦ China),radiation, and andthe test 4 /minload and scanning load time rate. were The 1.961 Vickers N and hardness 10 s, respectively. was measured The tensile on shear the cross test was section performed of the usingwelded CSS-44100 joint using material an MH-3 testing microhardness equipment testwith machine a 100 kN (Shanghai maximum Tuming, load (China Shanghai, Mechanical China), andTesting the Equipmenttest load and Co., loadLtd., Changchun, time were 1.961China) N, and and the 10 strain s, respectively. speed was 6 mm/min. The tensile shear test was performed using CSS-44100 material testing equipment with a 100 kN maximum load (China Mechanical Testing 3.Equipment Results and Co., Discussion Ltd., Changchun, China), and the strain speed was 6 mm/min.

3.1.3. Results Weld Formation and Discussion

3.1.1.3.1. Weld Weld Formation Appearance

3.1.1.The Weld resistance Appearance spot weld appearance depends on the coupling effect of heat, electric, and load, which will influence the welded joint quality, corrosion resistance, and look. Figure 3 shows the The resistance spot weld appearance depends on the coupling effect of heat, electric, and load, appearances of typical resistance spot welded joints, which were gained with different welding which will inﬂuence the welded joint quality, corrosion resistance, and look. Figure3 shows the currents. During the resistance spot welding process, while the welding current was 8.5 kA~10.5 kA, appearances of typical resistance spot welded joints, which were gained with different welding currents. there was no welding spatter on the spot weld surface. Until the welding current was 11 kA, the During the resistance spot welding process, while the welding current was 8.5 kA~10.5 kA, there was welding spatters were generated, and the welding spatters obviously increased. The welding spatter no welding spatter on the spot weld surface. Until the welding current was 11 kA, the welding spatters is caused by the large welding heat input, which results in the faster speed of melting metal than the were generated, and the welding spatters obviously increased. The welding spatter is caused by the expansion speed of the plastic ring, and the melted liquid metals fly out of the plastic ring. Because large welding heat input, which results in the faster speed of melting metal than the expansion speed of the welding current is the main parameter to determine the heat input (Q = i2Rt, Q is the welding heat the plastic ring, and the melted liquid metals ﬂy out of the plastic ring. Because the welding current is input, i is the welding current, and t the is welding time), the welding heat input increases sharply the main parameter to determine the heat input (Q = i2Rt, Q is the welding heat input, i is the welding when the welding current is increased, then the high heat input brings about more welding spatters. current, and t the is welding time), the welding heat input increases sharply when the welding current is increased, then the high heat input brings about more welding spatters.

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(a) (b)

FigureFigure 3. 3.Resistance Resistance spot weld weld( cappearance) appearance gained gained with withdifferent different welding(d) welding currents: currents:(a) 8.5 kA; ( ba) 10.0 8.5 kA; (b) 10.0kA; ( kA;c) 11.0 (c) kA; 11.0 (d kA;) 12.0 (d kA.) 12.0 kA. Figure 3. Resistance spot weld appearance gained with different welding currents: (a) 8.5 kA; (b) 10.0 kA; (c) 11.0 kA; (d) 12.0 kA. FigureFigure4 indicates 4 indicates the the three three feature feature zones zones onon thethe resistanceresistance spot spot weld weld surface, surface, and and it comprises it comprises of three circle zones (circle zone I, circle zone II, and circle zone III) from the center to the outside. of three circle zones (circle zone I, circle zone II, and circle zone III) from the center to the outside. TheFigure circle zone 4 indicates I was the the area three where feature the electrodes zones on contactedthe resistance with thespot base weld metal surface, during and the it resistance comprises The circle zone I was the area where the electrodes contacted with the base metal during the resistance ofspot three welding circle process,zones (circle and is zone located I, circle in the zone centre II, of and the circlespot weld zone surface. III) from It was the producedcenter to theby various outside. spot welding process, and is located in the centre of the spot weld surface. It was produced by various Thephysical circle zonefactors, I was such the as area the whereheat, electricity, the electrodes and load, contacted of the with electrodes the base and metal the bindingduring the force resistance of the physicalspotbase welding metal. factors, Becauseprocess, such asof and thethe is heat,highest located electricity, temperature in the centre and in of load, thethe circlespot of the weld zone electrodes surface.I and the It and lowwas the meltingproduced binding point by force variousof Zn of the basephysical(692 metal. K), factors, the Because welding such of asheat the the highest causedheat, electricity, the temperature Zn layer and toload, in melt the of and circlethe electrodesbezone squeezed I and and away thethe lowbindingby the melting electrodes, force pointof the of Znbase (692meanwhile metal. K), the Because the welding base of metal the heat highestsubstrat caused temperaturee was the Znexposed layer in and the to melt oxidized.circle and zone beThe I squeezedand circle the zone low away II melting is located by the point outside electrodes, of Zn meanwhile(692the K),adjacent the the welding baseregion metal of heat the substrate causedcircle zone the was I.Zn exposedThe layer profile to and meltof oxidized.the and circle be zonesqueezed The circleⅡ depended away zone by II onis the the located electrodes, working outside themeanwhilesurface adjacent shape regionthe ofbase the of metal electrodes the circle substrat and zonee the was I. indentation The exposed proﬁle and de ofpth oxidized. the of the circle spot The zone welded circle II dependedzone joint. II On is thelocated on circle the outside workingzone surfacetheII, adjacentthe shape Zn extruded ofregion the electrodes offrom the circle circle and zone zone the Ⅰ along indentationI. The the profile edge depth of thethe of electrodes,circle the spot zone weldedand Ⅱ depended molten joint. Zn Onon on the circle working circle zone zone II,surface theII was Zn shape extrudedaggregated of the from by electrodes the circle action zone and of gravitythe I along indentation and the surfac edge dee ofpth tension, the of electrodes,the then spot solidified welded and moltenjoint. to form On Zn thethe on Zncircle circle island zone zone II wasII,as the shown aggregated Zn extruded in Figure by from the4. The action circle SEM zone of image gravity Ⅰ along and and element the surfaceedge (white of tension,the spots electrodes, in then Figure solidiﬁed and 5b–d) molten map to Zn form distribution on thecircle Zn zone islandof asII shownthe was Zn aggregated inisland Figure (on4 by.the The the circle SEM action zone image of II gravity in and Figure elementand 4 surfacas an (white example)e tension, spots are then in shown Figure solidified in5 b–d)Figure to mapform 5, and distributionthe the Zn mainisland of components of the Zn island comprised of O (8.57%), Fe (15.44%), and Zn (65.24%). The circle zone theas Zn shown island in (onFigure the 4. circle The SEM zone image II in Figure and element4 as an (white example) spots are in Figure shown 5b–d) in Figure map5 distribution, and the main of III was the heat affected zone of spot welded joints. The base metal in this zone was heated and the componentsthe Zn island of the(on Zn the island circle comprisedzone II in Figure of O (8.57%), 4 as an Feexample) (15.44%), areand shown Zn (65.24%).in Figure 5, The and circle the zonemain III componentsZn layer was of oxidized the Zn island to form comprised ZnO. The ofZn O isla (8.57%),nd also Fe could (15.44%), generate and on Zn the (65.24%). circle zone The III. circle zone was the heat affected zone of spot welded joints. The base metal in this zone was heated and the Zn III was the heat affected zone of spot welded joints. The base metal in this zone was heated and the layer was oxidized to form ZnO. The Zn island also could generate on the circle zone III. Zn layer was oxidized to form ZnO. The Zn island also could generate on the circle zone III.

Figure 4. Three feature zones on the resistance spot weld surface.

FigureFigure 4. 4.Three Three featurefeature zoneszones on the resistance resistance spot spot weld weld surface. surface.

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(a) (b)

Figure 5. SEM image and elementelement mapmapdistribution distribution of of Zn Zn island: island: (a ()a SEM) SEM image; image; (b ()OKb) O αKα1; 1;(c )(c Fe) Fe K αKα1; 1;(d )(d Zn) Zn Kα K1.α 1.

From Figure 33a,a, duedue toto thethe lowlow weldingwelding current,current, thethe ZnZn onon thethe surfacesurface ofof circlecircle zonezone II waswas partiallypartially melted and the color was not much different from that on the base metal; the profile profile of circle zone II was small with a smooth transition, and the Zn island was generated in the circle zone III. While the welding current was 10.0 kA, the Zn layer on the the surf surfaceace of of the the circle circle zone zone was was not not seriously seriously damaged, damaged, and the the inner inner and and outer outer colors colors of of circle circle zone zone II IIwere were quite quite different. different. The The Zn Znisland island was was formed formed on the on adjacentthe adjacent part partof circle of circle zone zone I, because I, because the Zn the laye Znr layer on the on circle the circle zone zone II was II wasmelted melted and extruded, and extruded, and thenand thencooled cooled and andcrystallized crystallized on the on theoutside. outside. There There was was a Zn a Zn island island on on the the circle circle zone zone III. III. When When the welding current continued to to increase increase to to 11.0 11.0 kA, kA, the the Zn layer on the circle zone I was seriously damaged. There There was was a aCu-Zn Cu-Zn alloy alloy formed formed by by the the reac reactiontion of ofthe the molten molten Zn Znlayer layer and and copper copper on the on edgethe edge of circle of circle zone zone II, and II, andthe electrodes’ the electrodes’ adhesion adhesion occurred. occurred. When When the welding the welding current current reached reached 12.0 kA,12.0 the kA, appearance the appearance quality quality of the of thespot spot welded welded joint joint decreased decreased obviously obviously and and many many spatters spatters were generated because because of of the uneven heatheat distribution of the electrodes.electrodes. Figure 66 indicatesindicates thethe relationrelation between the the welding current current and and the the diameters of of the the three feature zones zones on on the weld appearance. When the the welding welding current current was was 8.5~9.5 8.5~9.5 kA, kA, the the diameter diameterss of circle of circle zone zone I were I werealmost almost unchanged. unchanged. With theWith continued the continued increase increase of the welding of the welding current, current,the diameters the diameters of circle zone of circle I increased zone I to increased the maximum to the atmaximum first and then at first decreased and then gradually. decreased The gradually. diameter The of diameterthe circle zone of the IIcircle changed zone little II changed with a low little welding with a current,low welding which current, was the which largest was with the a largest10.5 kA with current. a 10.5 Th kAe diameter current. of The the diameter circle zone of III the increased circle zone with III theincreased increase with of the the current increase from of the 8.5 current kA to 10.5 from kA, 8.5 but kA when to 10.5 the kA, welding but when current the weldingwas higher current than was10.5 kA,higher the than diameter 10.5 kA, of circle the diameter zone III of did circle not zone change III did obviously. not change The obviously. results di Thesplayed results that displayed the welding that currentthe welding had an current important had aninfluence important on influencethe weld appearance, on the weld and appearance, a low welding and a lowcurrent welding was used current on thewas basis used of on meeting the basis the of strength meeting requirements the strength of requirements the welded ofjoint. the welded joint.

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FigureFigure 6. 6. EffectEffectEffect ofof thethe weldingwelding currentcurrent onon sizessizes ofof threethree featurefeature zones zones on on the the weld weld appearance. appearance.

3.1.2. Main Main Dimensions Dimensions of of Welded Welded Joint Cross-Section The main dimensions of the resistance spot welded joint cross-section include the indentation (usually(usually expressedexpressed expressed byby by indentationindentation indentation rate,rate, rate, D/D/ D/δ))δ andand) and weldweld weld nuggetnugget nugget widthwidth width atat atthethe the overlapoverlap overlap surfacesurface surface (W),(W), (W), asas shownas shown in Figure in Figure 7. 7The. The indentation indentation influences inﬂuences th thee weld weld appearance appearance smoothness, smoothness, reduces reduces the weldedwelded jointjoint cross-sectioncross-section size,size, andand causescauses stressstress concentration,concentration, whichwhich resultsresults inin reducingreducing thethe strengthstrength of of the the welded joint. The tensile strength of the resistance spot weldedwelded jointjoint isis mainlymainly controlledcontrolled byby thethe W.W.

FigureFigure 7. 7. SchematicSchematicSchematic diagramdiagram ofof thethe mainmain dimensionsdimensions ofof thethe weldedwelded joint joint cross-section. cross-section.

The effect of the welding current on the indentationindentation rate is indicatedindicated in Figure 88a.a. TheThe resultsresults indicatedindicated thatthat thethe indentationindentation raterate waswas smallsmall andand increasedincreased lessless withwith aa lowlow welding welding currentcurrent because because thethe welding welding heat heat input input was was small, small, which which caused caused a a smallsmall amountamount ofof basebase metalmetal melting.melting.melting. When When the the welding current was between 9.5 kA and and 11.0 11.0 kA, kA, the the welding welding heat heat input input increased increased rapidly, rapidly, so more base metal was melted and the indentation rate in increased.creased. If If the the welding current was greater than 11.0 kA, the indentation rate increased gradually du duee to welding spatters and other defects, and the indentationindentation waswas too too serious serious toto satisfysatisfy thethethe weldingweldingwelding qualityqualityquality requirements.requirements.requirements. FigureFigure Figure 88b8bb displaysdisplaysdisplays thethethe relationshiprelationship between between the the welding welding current current and and the the W. W. The weld nugget width width increased increased rapidly rapidly from from 7.36 mm to 8.64 mm with an an increase increase of the current from 8.5 kA to 10.0 kA, and the maximum value was 8.75 8.75 mm mm at at the the 10.5 10.5 kA kA welding welding current. current. While While the the current current changed changed from from 10.0 10.0 kA to kA 10.5 to 10.5kA, the kA, weldingthe welding heat heatinput input reached reached a quasi-steady a quasi-steady state stateand the and change the change of the ofweld the nugget weld nugget width widthwas small. was Whensmall. Whenthe welding the welding current current was greater was greater than than 10.5 10.5 kA, kA, the thecurrent current density density was was higher, higher, and and a a large large number of welding spatters were were generated, which which reduced reduced the the amount amount of of melted melted base base metal metal in in the weld nugget and thus decreased the weld nugget widt width.h. If If the the welding welding current current continued continued to to increase increase toto 12.012.0 kA,kA, thethe weldweld nuggetnugget widthwidth increasedincreased becausebecause ofof thethe largerlarger weldingwelding heatheat input,input, butbut therethere werewere many welding spatters, and also some shrinkage and crack defects in the weldweld nugget.nugget.

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18 10.0

17 Materials 2018, 11, x FOR PEER REVIEW 9.5 7 of 13 16

15 9.0 18 10.0 14 )/% 17

δ 8.5 9.5 13 16

12 15 8.09.0

11 14 )/% 7.5

δ 8.5 10 13

9 12 7.08.0 Indentation rate(D/ 8 11 6.57.5 7 10

6 9 surface(W)/mm faying at width nugget Weld 6.07.0

Indentation rate(D/ 8.0 8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0 12.5 8.0 8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0 12.5 8 Welding current/kA Welding current/kA 6.5 7 (a) (b)

6 surface(W)/mm faying at width nugget Weld 6.0 8.0 8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0 12.5 8.0 8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0 12.5 FigureFigure 8. Relationships 8. RelationshipsWelding between between current/kA the the welding welding current and and (a (a) )indentation indentation Weldingrate rate (D/ current/kA (D/δ); (δb);) weld (b) weld nugget nugget widthwidth at overlap at overlap surface surface(a (W).) (W). (b)

3.2. Microstructure3.2. MicrostructureFigure 8. Relationships between the welding current and (a) indentation rate (D/δ); (b) weld nugget width at overlap surface (W). DueDue to the to uneventhe uneven heat heat input input and differentand different cooling cooling conditions, conditions, the microstructure the microstructure of the of resistance the weldedresistance3.2. joints Microstructure welded was very joints inhomogeneous. was very inhomogeneous. As shown As in shown Figure in9 Figurea, the 9a, whole the whole resistance resistance spot spot welded jointwelded comprisedDue joint to comprised ofthe the uneven base of metaltheheat base input (a metal zone), and (a uneven differentzone), un structurecoevenoling structure conditions, zone zone (b zone), the(b zone), microstructure fine fine grain grain zone zoneof the (c (c zone), zone), superheated zone (d zone), and weld nugget zone (e zone). The microstructure of the base superheatedresistance zone welded (d zone),joints was and very weld inhomogeneous. nugget zone As (e shown zone). in TheFigure microstructure 9a, the whole resistance of the base spot metal metal consists of ferrite and martensite as shown in Figure 9b. The grain and microstructure in the consistswelded of ferrite joint comprised and martensite of the base as shown metal (a in zone), Figure un9evenb. The structure grain andzone microstructure(b zone), fine grain in zone the uneven (c uneven structure zone were obviously heterogeneous, as shown in Figure 9c. Because the structurezone), zone superheated were obviously zone (d heterogeneous,zone), and weld asnugget shown zone in (e Figure zone).9 c.The Because microstructure the temperature of the base in this temperaturemetal consists in thisof ferrite zone andwas martensitebetween Ac as1 shownand Ac in3 during Figure the9b. resistanceThe grain andspot microstructurewelding process, in thethe zone was between Ac and Ac during the resistance spot welding process, the phase transformation phaseuneven transformation structure 1 zone and wererecrystallization3 obviously occurredheterogeneous, for part as of shown the base in metal,Figure and 9c. the Because fine ferrite the and recrystallization occurred for part of the base metal, and the fine ferrite and martensite formed; andtemperature martensite in formed; this zone meanwhile, was between the ferrite,Ac1 and which Ac3 during was not the austenitized, resistance spot became welding the coarse process, ferrite. the meanwhile,Therefore,phase transformation the there ferrite, were which alsoand similaritiesrecrystallization was not austenitized, to the occurred structure became for of part the theof base the coarse metalbase ferrite.metal, in this and Therefore,zone. the Duringfine ferrite there the were alsowelding similaritiesand martensite process, to theformed; the structure base meanwhile, metal of in the the the base fine ferrite, metalgrain which zone in this was was zone.not heated austenitized, During to above the became Ac welding3, and the allcoarse process, ferrite ferrite. and the base metalmartensiteTherefore, in the fine were there grain recrystallized were zone also was similarities heatedto austenite. to to above the The stru Acfine3cture, and of homogeneous all the ferrite base andmetal ferrite martensite in this and zone. martensite were During recrystallized were the to austenite.obtainedwelding The afterprocess, fine cooling, andthe base homogeneouswhich metal were in thesimilar ferrite fine tograin and the zone martensitenormalized was heated weremicrostructure to obtained above Ac afterof3, andheat cooling, all treatment, ferrite which and as were similarshownmartensite to the in Figure normalized were 9d. recrystallized The microstructure overheated to austenite. zone of consisted heat The treatment, fine of and coarse homogeneous as lath shown martensite in Figureferrite and 9andd. a littleThe martensite overheatedof the ferritewere zone shown in Figure 9e. The temperature in this zone was between 1100° and the solidus temperature, consistedobtained of coarse after lathcooling, martensite which were and similar a little ofto the ferritenormalized shown microstructure in Figure9e. of The heat temperature treatment, as in this andshown the austenite in Figure was9d. The overheated. overheated The zone grain consisted grew up of seriously coarse lath and martensite the chemical and compositionsa little of the ferrite in the zone was between 1100◦ and the solidus temperature, and the austenite was overheated. The grain grainshown were in uniform,Figure 9e. hence, The temperature the coarse martensite in this zone was was obtained between after 1100° rapid and cooling.the solidus The temperature, nugget zone grewcomprisedand up the seriously austenite of coarse and was lath theoverheated. martensite chemical The and compositions grain a little grew of upferrite inseriously thewith grain theand columnar the were chemical uniform, crystal compositions morphology, hence, in the the as coarse martensiteshowngrain werein was Figure obtaineduniform, 9f. Because hence, after rapid thethe coarsemaximum cooling. martensite Thetemperature nugget was obtained zonegradient comprised after in rapidthe nugget ofcooling. coarse zone The lath was nugget martensite along zone the and a littleaxiscomprised of of ferrite the electrodes, of with coarse the laththe columnar moltenmartensite liquid crystal and metal morphology,a little nuclea of ferriteted asand with shown crystallized the columnar in Figure at the 9crystalf. fusion Because morphology, line the first, maximum and as temperaturethenshown formed in gradient Figure the columnar 9f. in Because the nugget austenite the maximum zone along was the temperature along direct theion axisof gradient the of higher the in electrodes, the temperature nugget thezone gradient. molten was along liquidFinally, the metal nucleatedtheaxis solid of and the austenite crystallizedelectrodes, transformed the at molten the fusion into liquid martensite line metal first, nuclea be andcauseted then and of formed thecrystallized rapid the cooling columnarat the fusionrate austeniteof line the first, welding alongand the directionprocessthen offormed and the low higher the carbon columnar temperature component austenite gradient. in along the base the Finally, directmetal.ion the of solid the higher austenite temperature transformed gradient. into Finally, martensite becausethe ofsolid the austenite rapid cooling transformed rate of into the weldingmartensite process because and of lowthe rapid carbon cooling component rate of inthe the welding basemetal. process and low carbon component in the base metal.

(a) (b)

Figure 9. Cont. Materials 2018, 11, 2310 8 of 13

Materials 2018, 11, x FOR PEER REVIEW 8 of 13

(e) (f)

FigureFigure 9. Microstructure 9. Microstructure of (a) of different (a) different zones zones of the of welded the welded joint; joint; (b) base (b) base metal; metal; (c) uneven (c) uneven structure structure zone; (d) fine grain zone; (e) superheated zone; (f) weld nugget zone. zone; (d) fine grain zone; (e)(e superheated) zone; (f) weld nugget zone. (f) The weldTheFigure weld nugget 9. Microstructurenugget is the is the important important of (a) different zone zone of zonesof resistanceresistan of thece welded spot spot welded joint; welded ( bjoints,) base joints, andmetal; and its ( cmicrostructure its) uneven microstructure characteristicscharacteristicsstructure directly directlyzone; affected (d )affected fine grain the themechanical zone; mechanical (e) superheated propertiesproperties zone; of of welded ( weldedf) weld joints. nugget joints. The zone. The heat heat generation generation and and heat transferheat transfer in the nuggetin the nugget zone were zone different were different under different under different welding currents,welding currents, so the microstructure so the microstructureThe weld nuggetwas very is the different. important Figure zone of10 resistanshows cethe spot influence welded joints,of a typical and its currentmicrostructure on the was very different. Figure 10 shows the influence of a typical current on the microstructure of the microstructurecharacteristics directlyof the weld affected nugget. the It mechanical can be seen prop thaterties while of the welded welding joints. current The was heat low, generation the welding and weld nugget.heatheat inputtransfer It canwas in below, the seen and nugget that the while weldzone nuggetthewere welding differentwas mainly current under lath was differentmartensite low, the welding with welding a finecurrents, heat and input uniformso the was low, and thestructure.microstructure weld nugget At the wassame was very time, mainly different. the lathplastic martensite Figure deformation 10 withshows of the a finethe weld influence and nugget uniform zoneof a structure.wastypical large, current and At thethere on same wasthe time, the plasticnomicrostructure welding deformation defect of thein the of weld theweld nugget. weld nugget nuggetIt can zone. be zoneWhileseen that wasthe while welding large, the and weldingcurrent there was current was 10.5 nowas kA, welding low,the welding the welding defect heat in the weld nuggetinputheat inputincreased zone. was While andlow, the and the microstructure weldingthe weld currentnugget became was was coarser. mainly 10.5 kA, The laththe decrease martensite welding of the with heat cooling inputa fine rate increasedand resulted uniform andin the microstructurethestructure. decrease At became ofthe martensite same coarser. time, and the The plasticthe decrease increase deformation of ofthe ferrite. coolingof the The weld ratemicrostructure nugget resulted zone in wasof the the large,decrease weld and nugget there of martensite waswas no welding defect in the weld nugget zone. While the welding current was 10.5 kA, the welding heat and therelatively increase uniform, of ferrite. and The there microstructure was no obvious of the welding weld nugget defect. was If the relatively welding uniform, current andwas therethe was input increased and the microstructure became coarser. The decrease of the cooling rate resulted in no obviousmaximum welding value defect.of 12.0 IfkA the, a weldinglarge number current of spatters was the caused maximum some valueheat loss of 12.0and a kA, rapid a large cooling number rate,the decreaseso that theof martensiteweld nugget and microstructure the increase of cons ferrite.isted The of lathmicrostructure martensite. of The the grains weld nuggetin the weldwas of spatters caused some heat loss and a rapid cooling rate, so that the weld nugget microstructure nuggetrelatively center uniform, grew upand and there coarsened was no greatly, obvious as weldingshown in defect. Figure If 10c. the In welding short, when current the waswelding the consistedcurrentmaximum of varied lath value martensite. from of 8.512.0 kA kA to The, 12a large kA, grains the number weld in the nuggof weldspatterset columnar nugget caused structure center some heat grew gradually loss up and and coarsened, a coarsenedrapid cooling mainly greatly, as shownbecauserate, inso Figureofthat the the larger 10 weldc. heat In nugget short, input microstructure and when the the reduction welding cons ofisted the current coolingof lath varied martensite.rate. from The 8.5 grains kA to in 12 the kA, weld the weld nuggetnugget columnar center structuregrew up and gradually coarsened coarsened, greatly, as shown mainly in becauseFigure 10c. of In the short, larger when heat the input welding and the reductioncurrent of thevaried cooling from 8.5 rate. kA to 12 kA, the weld nugget columnar structure gradually coarsened, mainly because of the larger heat input and the reduction of the cooling rate.

(a) (b) (c)

Figure 10. Effect of the typical welding current on the microstructure of the weld nugget: (a) 8.5 kA;

(b) 10.5 kA; (c) 12 kA. Materials 2018, 11, x FOR PEER REVIEW 9 of 13

MaterialsFigure2018 10., 11 Effect, 2310 of the typical welding current on the microstructure of the weld nugget: (a) 8.5 kA; 9 of 13 (b) 10.5 kA; (c) 12 kA.

3.3.3.3. Microhardness Microhardness Distribution Distribution TheThe microhardness measurementmeasurement was was performed performed on on the the cross cross section section of the of resistance the resistance spot welded spot weldedjoint. The joint. schematic The schematic diagram diagram of the measurement of the measurem locationent location is shown is in shown Figure in 11 Figure. The distance11. The distance between betweentwo test two points test in points the base in the metal base andmetal weld and nugget weld nugget zone waszone 0.5 was mm. 0.5 mm. Because Because the HAZ the HAZ width width was wasnarrow, narrow, the twothe two test pointstest points distance distance was was 0.25 0.25 mm mm in HAZ. in HAZ.

FigureFigure 11. 11. SchematicSchematic diagram diagram of of the the microhardness microhardness me measurementasurement location location of of the the welded welded joint. joint.

FigureFigure 12 12 displays displays the the microhardness microhardness distributions distributions of of resistance resistance spot spot welded welded joints joints gained gained with with differentdifferent weldingwelding currents. currents. The The microhardness microhardness of the of base the metal base was metal 198 HV was0.2, and198 theHV microhardness0.2, and the microhardnessin HAZ was obviously in HAZ higherwas obviously than the higher base metal. than Thethe base microhardness metal. The of micr theohardness weld nugget of the zone weld was nuggetthe highest, zone was which the washighes abovet, which 350 HVwas0.2 above. The 350 microhardness HV0.2. The microhardness at the edge of atthe the weldedge of nugget the weld was nuggetslightly was higher slightly than higher that of than the that weld of nuggetthe weld center. nugget Due center. to the Due uneven to the uneven distribution distribution of the current of the currentdensity density at the welding at the welding joint, the joint, current the current density density at the edge at the of edge the weld of the nugget weld wasnugget greater was thangreater the thanaverage the average current density,current whichdensity, generated which generated a greater a resistance greater resistance heat. Therefore, heat. Therefore, during the during resistance the resistancespot welding spot process, welding the process, solidification the solidification and crystallization and crystallization first occurred first at theoccurred edge ofat thethe weldedge nugget.of the weldMeanwhile, nugget. theMeanwhile, temperature the gradienttemperature was largegradient and was the coolinglarge and rate the was cooling fast, sorate the was martensite fast, so wasthe martensitelarge and coarsewas large and and the coarse microhardness and the microhardne was higherss in was this higher zone. Within this the zone. increase With the of the increase welding of thecurrent, welding the averagecurrent, microhardnessthe average microhardness in the weld nugget in the zone weld decreased. nugget zone While decreased. the welding While current the weldingwas 8.5 current kA, the was microhardness 8.5 kA, the microhardness of the weld nugget of the wasweld higher nugget because was higher of the because faster of cooling the faster rate, coolingand the rate, microstructure and the microstructure was lath martensite, was lath martensite, which was uniformwhich was and uniform fine, so and the fine, hardness so the was hardness higher. wasWhile higher. the welding While the current welding was current 10.5 kA, was the cooling10.5 kA, rate the shouldcooling decrease. rate should The decrease. columnar The crystals columnar of the crystalsweld nugget of the grewweld up,nugget mainly grew composed up, mainly of lathcomposed martensite of lath and martensite acicular ferrite, and acicular which resultedferrite, which in the resulteddecrease in of the hardness. decrease When of hardness. the welding When current the weld continueding current to increase continued to 12.0 tokA, increase the microhardness to 12.0 kA, the of microhardnessmost zones of theof most weld zones nugget of wasthe weld higher nugget than thatwas ofhigher 10.5 kAthan due that to of the 10.5 large kA heatdue inputto the andlarge coarse heat inputstructure, and coarse but there structure, were welding but there defects were in welding the center defects of the in weld the nugget,center of so the that weld the averagenugget, valueso that of themicrohardness averageMaterials 2018value, 11 decreased., ofx FOR microhardness PEER REVIEW decreased. 10 of 13

450 8.5 kA 10.5 kA 400 12.0 kA

350 0.2

300

250 Microhardness/HV

200

150 -10 -8 -6 -4 -2 0 2 4 6 8 10 Distance from weld nugget center/mm Figure 12. Microhardness distributions of welded joints with different welding currents. Figure 12. Microhardness distributions of welded joints with different welding currents. 3.4. Tensile Shear Strength The tensile shear force is often used to characterize the welded joint strength. The larger the tensile shear force is, the better the strength is. Figure 13 indicates the effect of the welding current on tensile shear force. Compared with Figure 8b, it can be found that the variation trend of the W and tensile shear force with the welding current is basically the same; that is, the larger the weld nugget width, the greater the tensile shear force. With the first increase of welding current, the weld nugget width and tensile shear force all obviously increased. While it was 10.5 kA, the weld nugget width and tensile shear force all reached the maximum value of 8.75 mm and 24.20 kN, respectively. Subsequently, the weld nugget width dropped rapidly and an inflection point appeared while the welding current was 11.5 kA, but the tensile force decreased continuously. With the large welding current, the melting amount of the base metal increased gradually, which caused the increase of the weld nugget width. Thus, the bonding strength of the resistance spot welded joint increased and the tensile shear force increased. However, when the welding current was 12.0 kA, the welding heat input was too large, resulting in a large number of spatters, shrinkage cracks, and other defects in the welded joint. Although the weld nugget width increased, the effective bonding width of the welded joint decreased, so the tensile shear force decreased.

Tensile shear force/kN 18

16 8.0 8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0 12.5 Welding current/kA Figure 13. Effect of the welding current on the tensile shear force of welded joint.

During the tensile shear experiment, there were two failure modes: Interface failure and pullout failure, as shown in Figure 14. The SEM images of fracture surfaces were indicated in Figure 15. The

Materials 2018, 11, x FOR PEER REVIEW 10 of 13

450 8.5 kA 10.5 kA 400 12.0 kA

350 0.2

300

250 Microhardness/HV

200

150 -10 -8 -6 -4 -2 0 2 4 6 8 10 Distance from weld nugget center/mm Materials 2018, 11, 2310 10 of 13 Figure 12. Microhardness distributions of welded joints with different welding currents.

3.4. Tensile Shear Strength The tensile shear force is often used to characterizecharacterize the weldedwelded jointjoint strength.strength. The larger the tensile shear force is, the better the strength is. Figu Figurere 13 indicates the effect of the welding current on tensile tensile shear shear force. force. Compared Compared with with Figure Figure 8b,8b, it itcan can be be found found that that the the variation variation trend trend of the of theW and W tensileand tensile shear shear force forcewith the with welding the welding current current is basi iscally basically the same; the that same; is, thatthe larger is, the the larger weld the nugget weld width,nugget the width, greater the the greater tensile the shear tensile force. shear With force. the With first theincrease ﬁrst increase of welding of welding current,current, the weld the nugget weld widthnugget and width tensile and shear tensile force shear all force obviously all obviously increase increased.d. While it While was 10.5 it was kA, 10.5 the kA, weld the nugget weld nugget width widthand tensile and tensile shear shearforce forceall reached all reached the maximum the maximum value value of 8.75 of 8.75 mm mm and and 24.20 24.20 kN, kN, respectively. respectively. Subsequently, the weld nugget widthwidth dropped rapidly and an inﬂectioninflection point appeared while the welding current was 11.5 kA, but the tensile forceforce decreased continuously. With the large welding current, the melting amount of the base metal increased gradually, which caused the increase of the weld nugget width. Thus, the bonding strength of th thee resistance spot welded joint increased and the tensile shearshear forceforce increased. increased. However, However, when when the the welding welding current current was was 12.0 12.0 kA, thekA, welding the welding heat input heat wasinput too was large, too resultinglarge, resulting in a large in a number large number of spatters, of spatters, shrinkage shrinkage cracks, andcracks, other and defects other indefects the welded in the weldedjoint. Although joint. Although the weld the nugget weld nugget width width increased, increased, the effective the effective bonding bonding width width of the of welded the welded joint jointdecreased, decreased, so the so tensile the tensile shear shear force force decreased. decreased.

Tensile shear force/kN 18

16 8.0 8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0 12.5 Welding current/kA Figure 13. EffectEffect of the welding current on the tensile shear force of welded joint.

During the tensile shear experiment, there were two failure modes: Interface failure and pullout failure, as as shown shown in in Figure Figure 14. 14 The. The SEM SEM images images of fracture of fracture surfaces surfaces were were indicated indicated in Figure in Figure 15. The 15. The welded joints, which were gained with the lower welding current (≤9.5 kA) and higher welding current (≥11 kA), always ruptured along the overlap surface, as shown in Figure 14a. While the welding current was low (≤9.5 kA), the W was narrow, as well as the strength of the base metal was high. Therefore, under the tensile shear force, the crack produced on the weld nugget edge at the overlap surface at first and then extended along the overlap surface until the welded joint failed with the interface failure mode. While the welding current was high (≥11 kA), the bonding strength of welded joints was small due to the welding defects, such as spatter, shrinkage, and cracks, in the weld nugget, which resulted in the smaller tensile shear force for the welded joint, and its tensile shear specimen also ruptured along the overlap surface. As shown in Figure 15a, the river pattern was obvious on the fracture surface, which illustrated the brittle fracture characteristics. While the welding current was 10 kA, the fracture pattern of the welded joint was the pullout failure mode, as shown in Figure 14b. It can be found that the fracture surfaces are mainly dimples and a small amount of cleavage steps from Figure 15b. Under the tensile shear force, the tensile specimen first produced necking in the HAZ of the welded joint. With the increase of tensile shear force, the dimples grew up and converged, and then the tensile specimen broke down in the base metal. In the present study, the weld nugget width and tensile shear force were all maximum with a 10.5 kA welding current. Materials 2018,, 11,, xx FORFOR PEERPEER REVIEWREVIEW 11 of 13 welded joints, which were gained with the lower welding current (≤9.5 kA) and higher welding current (≥11 kA), always ruptured along the overlap surface, as shown in Figure 14a. While the welding current was low (≤9.5 kA), the W was narrow, as well as the strength of the base metal was high. Therefore, under the tensile shear force, the crack produced on the weld nugget edge at the overlap surface at first and then extended along the overlap surface until the welded joint failed with the interface failure mode. While the welding current was high (≥11 kA), the bonding strength of welded joints was small due to the welding defects, such as spatter, shrinkage, and cracks, in the weld nugget, which resulted in the smaller tensile shear force for the welded joint, and its tensile shear specimen also ruptured along the overlap surface. As shown in Figure 15a, the river pattern was obvious on the fracture surface, which illustrated the brittle fracture characteristics. While the welding current was 10 kA, the fracture pattern of thethe weldedwelded jointjoint waswas thethe pulloutpullout failurefailure mode,mode, asas shown in Figure 14b. It can be found that the fracture surfaces are mainly dimples and a small amount of cleavage steps from Figure 15b. Under the tensile shear force, the tensile specimen first produced necking in the HAZ of the welded joint. With the increase of tensile shear force, the dimples grew up neckingMaterials 2018 in the, 11, HAZ 2310 of the welded joint. With the increase of tensile shear force, the dimples grew11 of up 13 and converged, and then the tensile specimen broke down in the base metal. In the present study, the weld nugget width and tensile shear force were all maximum with a 10.5 kA welding current. During theDuring tensile the shear tensile test shear for welded test for joints welded produced joints producedwith a 10.5 with kA welding a 10.5 kA curre weldingnt, because current, of the because large weldof the nugget large weld and few nugget welding and few defects, welding the welded defects, jo theintintwelded waswas notnot joint easyeasy was toto rupturerupture not easy fromfrom to rupture thethe overlapoverlap from surface.the overlap The surface. tensile stress The tensile on the stress edge on of thethe edgeweldld of nuggetnugget the weld increasedincreased nugget gradually,gradually, increased gradually,owingowing toto thethe owing angleangle to betweenthe angle the between overlap the overlapsurface surfaceand tensile and tensileforce. force.The HAZ The HAZ was wasthe theweakest weakest zone zone due due to to the inhomogeneousinhomogeneous microstructure,microstructure, coarsecoarse grains,grains, andand lowlow plasticity plasticity and and toughness. toughness. WithWith With the the increase increase of tensile force, force, the the necking necking occurred occurred first first in in the the HA HAZ,Z, and and a number a number of ofmicro-holes micro-holes began began to toform form in thein the center center of the of thenecking. necking. The Themicro-holes micro-holes grew grew and formed and formed the dimples, the dimples, which which converged converged to form to theform crack. the crack. Finally, Finally, the crack the was crack torn was along torn the along HAZ the to HAZ formform to thethe form pulloutpullout the pullout teartear failure,failure, tear failure, asas showshow as inin show FigureFigure in 14c.Figure Figure 14c. Figure15c shows 15c showsthe pullout the pullout tear fracture tear fracture surface, surface, which which was mainly was mainly composed composed of small of small and uniformand uniform dimples. dimples.

(a) (b) (c)

Figure 14. ThreeThree failure failure modes modes of of the resistance spot welded joints: ( (a) Interface failure; ( b) pulloutpullout failure (base metal teartear fracture);fracture); ((cc)) pulloutpullout failurefailure (pullout(pullout teartear fracture).fracture).

(a) (b) (c)

Figure 15. SEMSEM imagesimages ofof thethe fracturefracture surface:surface: (( aa)) Interface Interface failure; failure; ( (bb)) pullout pullout failure failure (base (base metal metal tear tear fracture); ( c) pullout failure (pullout(pullout teartear fracture).fracture). 4. Conclusions

(1) The Zn island was generated on the resistance spot weld surface because of the melting and aggregating of the Zn layer under the action of heated and squeezed by the electrodes. While the welding current increased from 8.5 kA to 12.0 kA, the indentation rate kept growing to 16.5% due

to the increase of the welding heat input. However, the weld nugget width obviously increased at ﬁrst, which reached the maximum when the welding current was 10.5 kA, and then decreased. (2) The whole resistance spot welded joint comprises of the base metal, uneven structure zone, fine grain zone, superheated zone, and weld nugget zone. The nugget zone was mainly comprised coarse lath martensite and little ferrite with columnar crystal morphology due to the high temperature gradient and rapid cooling rate. With the increase of the welding current, the microstructure of the weld nugget became coarser; meanwhile, the martensite decreased and the ferrite increased. (3) The microhardness of the weld nugget zone was highest and the base metal was lowest. With the increase of the welding current, the average microhardness in the weld nugget zone decreased. While the welding current increased from 8.5 kA to 10.5 kA, the tensile shear force was obviously raised, owing to the increase of the weld nugget width. The tensile shear force reached the maximum value of 24.20 kN with a 10.5 kA welding current. If the welding current increased Materials 2018, 11, 2310 12 of 13

continuously, the tensile shear force decreased because of a large number of spatters and the high indentation rate. The failure modes mainly depended on the weld nugget width at the overlap surface and welding defect. Therefore, while the CR590T/340YDP galvanized dual phase steel sheets with 2 mm thickness were performed using resistance spot welding, the recommended welding current was 10.0~10.5 kA with a 20 cycles welding time and 4.0 kN electrode force.

Author Contributions: Writing, funding acquisition and investigation, X.Z.; conceptualization, methodology and supervision, Z.R.; validation and investigation, F.Y.; formal analysis and data curation, H.Y. Funding: This research was funded by Jilin Scientific and Technological Development Program (20160520055JH). Conflicts of Interest: The authors declare no conflict of interest.

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